In mobile telecommunication, electromagnetic waves in the radio frequency (RF) spectrum are used for communication between fixed network transceiver stations and user equipment (UE). An example of a mobile communication technology is the Universal Mobile Telecommunications System (UMTS). According to UMTS standards, user equipment and the base station transmit in respective radio frequency bands specified, e.g., by the 3rd Group Partnership Project (3GPP) in the Technical Specification TS 25.101, which is incorporated herein by reference.
User equipment and the fixed network transceiver stations, which are referred to as Node B in UMTS standards, do not use the same frequency reference. A respective internal reference-frequency unit is used on each side as a frequency reference. In other words, the reference-frequency units generate respective local reference signals with a reference frequency. The reference signal on the transmitter side is used in generating a RF carrier signal in the process of modulating symbols to be transmitted. The reference signal on the receiver side is used in demodulating (down-converting) received signals for recovering symbols.
The frequency reference in the receiver part of a user equipment must be capable of tuning to the frequency of received RF signals in order to perform a demodulation with an error rate as low as possible. Therefore, the frequency reference on the receiver side is a controllable reference-frequency unit. It can for instance be implemented in the form of a controllable oscillator such as a voltage-controllable oscillator (VCO).
An error in the reference frequency of the reference signal generated by the controllable reference-frequency unit will result in an increased error rate of reception. Therefore, frequency-offset estimation algorithms have been developed for use in controlling the reference frequency of the reference signal in order to reduce a frequency mismatch between the received RF signals and the reference signal. The higher the accuracy of frequency-offset estimation, the lower can be the requirements on the complexity of the controllable reference-frequency unit.
In UMTS, TS 25.101 requires that the modulated carrier frequency, i.e., the reference frequency used by the user equipment, shall be accurate to within ±0.1 PPM (Part Per Million) observed over a period of one timeslot, in comparison to the carrier frequency received from a Node B. To accommodate this requirement, the user equipment performs an automatic frequency control (AFC) loop, which is sometimes also referred to as automatic frequency correction loop. AFC serves to keep the reference frequency of the UE within the required error margin with respect to the frequency of the RF signals received from the assigned Node B.
According to TS 25.101, causes of a frequency error between received RF signals and the internally generated reference frequency of a user equipment are a frequency error on the side of the Node B and a Doppler shift. TS 25.101 requires that in the latter case, signals from a Node B must be averaged over a sufficient time so that errors due to noise or interference are allowed for only within the ±0.1 PPM figure.
US 2003/0054768 A1 describes a RF receiver device for acquiring and tracking pilots in a CDMA (Code Division Multiple Access) communication system such as Wideband CDMA (W-CDMA), which is used in UMTS technology. Frequency acquisition is performed by means of a Rake receiver for a number of signal instances in a multipath reception configuration. Frequency tracking of the different multipath signals is performed using a combination of a frequency control loop, which is maintained for a precision oscillator used for down-converting the received signal, and by using further frequency control loops for the different Rake-finger processors assigned to the acquired multipath signals. Each finger processor has an associated frequency-control loop for frequency control of a rotator used for complex multiplication of received complex data samples on the respective path.
The dual-loop frequency control scheme disclosed in this document is rather complex. It would be desirable to provide a RF receiver device with a simpler frequency-control mechanism.